In recent years, global scale environmental problems such as the whole global warming and destruction of ozone layer have been greatly closed up. In particular, a reduction of carbon dioxide emission which has a great influence upon the whole global warming has been a great concern in many countries.
As regards the reduction of carbon dioxide emission, a reduction of fuel consumption of automobiles is mentioned as one of such problems. Accordingly, a sliding member and a lubricating oil play an important role.
The sliding member is required to be excellent in wear resistance and to show a low friction coefficient relative to a sliding part of an engine which is exposed to severe environment in friction and wear. A hard thin film material has been recently increasingly used for such purposes.
Meanwhile, a diamond-like carbon (DLC) material is expected to serve as a low friction sliding material because of its lower coefficient of friction in the air in the absence of a lubricating oil as compared with wear resisting hard coating material such as TiN and CrN.
On the other hand, as a measure for fuel saving in lubricating oil, suggested are (1) to reduce a viscosity resistance in a hydrodynamic lubrication region and a stirring resistance in an engine by lowering the viscosity thereof and (2) to decrease a friction loss in a boundary lubrication region by compounding an optimum friction modifier and various additives. As the friction modifier, many studies have been made chiefly on organic molybdenum compounds such as MoDTC and MoDTP. In a conventional sliding surface made of a steel material, a lubricating oil compounded with an organic Mo compound exhibiting an excellent low friction coefficient at an early stage after the start of use has been used and proven to be effective.
It is reported, however, that an ordinary DLC material which shows excellent low friction properties in the air gives only a low level of friction reducing effect, when used for a sliding part in the presence of a lubricating oil (see, for example, Non-Patent Document 1). It is also known that satisfactory effect of reducing friction is not obtainable, when a lubricating oil composition containing an organic molybdenum compound is applied to a sliding part provided with such a DLC material (see, for example, Non-Patent Document 2).
In this circumstance, a technique is disclosed in which a lubricating oil composition containing a fatty acid ester-type or an aliphatic amine-type, ashless friction modifier is applied to a sliding surface between a DLC member and an iron-based member or a DLC member and an aluminum alloy member (see, for example, Patent Document 1 and Patent Document 2).
Even when such a lubricating oil composition containing a fatty acid ester-type or an aliphatic amine-type, ashless friction modifier is applied to a sliding part having a DLC member, the low friction characteristics and fuel saving effect are merely comparative to those attained by an organic molybdenum compound-containing lubricating oil composition and are therefore not fully satisfactory.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-238982
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-155891
[Non-Patent Document 1] Kano et al., “Japan Tribology Congress, Proceedings, Tokyo,” May 1999, p 11-12
[Non-Patent Document 2] Kano et al., “World Tribology Congress,” September 2001, Vienna, Proceedings, p 342